One example of exhaust gas purification device that captures the particulate matter emitted from a diesel engine by a filter which is called a diesel particulate filter is seen in a continuous regeneration type DPF device.
In the continuous regeneration device, when the exhaust gas temperature is equal to or higher than approximately 350° C., PM captured by a filter burns continuously to be purified and the filter is regenerated by itself. However, when the exhaust temperature is low in a case of low speed with low load, catalyst is not activated due to catalyst temperature drop and thereby it becomes difficult to perform self-regeneration of the filter through oxidation of PM. Owing to this, clogging is promoted due to accumulation of PM in the filter, resulting in a problem of back pressure increase caused by the clogging.
Therefore, when the accumulated amount of PM in the filter surpasses a predetermined amount (threshold), a forced regeneration requiring forced combustion removal of captured PM is carried out. In the forced regeneration, the exhaust gas temperature is raised forcibly via multi-injection (multistage delay injection) or post injection (after injection) and the like. That is, HC (hydrocarbon) is burnt by oxidation catalyst disposed upstream side of the filter or oxidation catalyst supported by the filter. Through utilization of the oxidation reaction heat, the exhaust gas temperature at the filter entrance or the filter surface is raised. The filter temperature is raised higher than the temperature, at which accumulated PM in the filter burns by this temperature rise, and thereby, the PM combustion removal is performed.
There are two cases in this forced regeneration, one is manual and the other is automatic. In a case of manual regeneration, when the clogging of the filter surpasses the predetermined amount, a warning is given to the driver and the driver who receives the warning pushes a button to start the forced regeneration to perform it. On the other hand, in a case of automatic, when the clogging of the filter surpasses a predetermined amount, no warning is given to the driver, but the forced regeneration is performed automatically even during driving.
As an example of such an exhaust emission purification system, for instance, just as Japanese Patent Application Kokai Publication 2005-256628 and the Japanese Patent Publication 3824003 disclose, such an exhaust emission purification system has been proposed that forced regeneration start timing is judged in manual or automatic regeneration, combining judgment by a detecting means of PM captured amount (differential pressure across DPF or elapse of a certain time duration) and judgment by mileage after the previous force regeneration operation of DPF.
In such an exhaust emission purification system, differential pressure across DPF and the mileage after a forced regeneration are used to judge whether the forced regeneration of DPF is necessary or not, and a control shown in FIG. 6 is carried out. In this control, the differential pressure across DPF ΔP is compared with the predetermined differential pressure across DPF threshold ΔPs and when ΔP≧ΔPs is satisfied, the procedure goes to the determination means of regeneration method. In addition, the mileage ΔM after forced regeneration is compared with the predetermined mileage threshold ΔMs and when ΔM≧ΔMs is satisfied, the procedure goes to the determination means of regeneration method. That is, when the differential pressure ΔM across DPF that rises with the increase of the accumulated amount of PM surpasses the predetermined differential pressure threshold ΔMs, or when the mileage ΔM after the previous regeneration of DPF surpasses the predetermined mileage threshold ΔMs, the forced regeneration is judged necessary. As for determination means of regeneration method, whether it should be manual or automatic is determined and the forced regeneration is carried out, if manual regeneration is the case, by means of button operation by the driver, and if automatic is the case, automatically.
As far as the differential pressure across DPF is concerned, it has been considered in the prior art that, as shown in FIG. 7, accompanying with increase of the mileage ΔM, the accumulated amount of PM ΔV increases and also the differential pressure across DPF ΔP rises. Consequently, it is assumed that when the accumulated amount of PM ΔV is increased, the differential pressure across DPF (ΔP) is monotonically increased. Note that in FIG. 7, solid line A shows an example of the case where there is hardly any uneven distribution of PM, dotted line B shows an example of the case where there is uneven distribution of PM and two-dot chain line C shows an example of the case where there is a lot of uneven distribution of PM.
However, as shown in FIG. 8, it has been discovered that the differential pressure across DPF ΔP is decreased depending on running pattern despite that the accumulated amount of PM ΔV is increased. The reason of which is considered that when mileage ΔM is increased gradually, PM is not accumulated evenly but accumulated unevenly, accordingly the accumulation does not appear in the differential pressure of DPF ΔP.
In such a case, conventional judgment having a constant threshold ΔP of the differential pressure across DPF can not perform correct judgment of starting of forced regeneration, and accordingly PM is captured by the DPF until the forced regeneration is judged to be necessary through determination using the mileage ΔM and the mileage threshold ΔMs. As a result, the forced regeneration could not be carried out through the differential pressure judgment, and thereby, the PM excessively accumulated amount ΔVa captured excessively comes to added on the PM accumulated amount ΔVs for which the forced regeneration is desired.
Consequently, in a case where the differential threshold ΔPs is kept constant, if the interval of forced regeneration is long, or in a long distance driving, PM accumulated amount ΔV may surpass the limitation at the time when the necessity of forced regeneration is judged by the mileage threshold ΔMs. In a case where the limitation is surpassed, the temperature in DPF is raised excessively, which may cause melting-loss of DPF in the worst case. That is to say, when the differential pressure threshold ΔPs to judge forced regeneration start timing is kept constant regardless the mileage ΔM, PM accumulated unevenly in the occasion of forced regeneration burns all at once and the temperature in DPF may rise excessively resulting in melting-loss of DPF.